Thursday, March 19, 2009

New tenants at your birdbath? Global warming may be to blame. See the
cardinal at Songbird Prairie Bed and Breakfast

RunnerJenny/Flickr Creative Commons

The cardinal was not found in Illinois until about 100 years ago, when population density forced it farther north, according to local bird surveys. Its story is an example of non-climate-related bird range shift.

When migratory birds arrive, they’re used to the fact that certain trees are budding out and certain insects are eating those tender leaves, because they eat those insects.

So what happens when people start screwing with those relationships?

Well, the trees start budding out earlier, so then the birds have to migrate out earlier. Or maybe they have to learn to rely on a whole different set of resources.

That puts extra stresses on them.

There’s a whole complicated set of relationships related to migratory birds and the resources they’re used to using.

We’re seeing that some birds are starting to migrate out earlier and some aren’t. That has nothing to do with this current Audubon study. But it’s another issue out there that it’s important to understand.

It happened so gradually you might have missed it.

For the past 40 years, birds have been making a nationwide run for the northern border, and a controversial recent study blames the warming climate.

In February, the Audubon Society released an analysis of data compiled over almost half a century that they say suggests that a slow, northbound shift of wintering grounds for almost every type of North American bird was directly linked to global warming.

More than 60 different bird species exchanged their current winter ranges -- the areas they settle in during colder months -- for new ones more than 100 miles north.

“It’s clear that if you look at the change in birds’ winter ranges,” said Judy Pollock, bird conservation director at Audubon Society Chicago region, “that climate change is having an affect. It just raises a million questions about everything that’s going on in the biological world.”

The study combined two sets of data to arrive at its conclusion that “while causation is nearly impossible to prove, global climate change is the most likely explanation” for the birds’ range shift.

The first data set was 40 years’ worth of figures from Audubon’s annual Christmas Bird Count, a methodical head-count of local bird numbers conducted by hundreds volunteers all over the country.

The results of each volunteer’s findings were aggregated over 40 years to reveal the range shift trend. The numbers revealed the presence of large numbers of bird species in areas they have not been found historically.

“If it was just one group, that might not be accurate,” said Joel Greenberg, author of A Natural History of the Chicago Region. “But if you look at Evanston, Madison and Toronto, you can at least say the findings are worth looking at more closely.”

“I know I’m politically incorrect on this, but it had an agenda,” Zick said of the study. “It wanted to show climate change affecting birds, so it did.”

Zick said many explanations exist for range shift other than climate change.

Some bird species, he said, might have begun including Chicago in their winter range to avoid the competition they find in warmer locales.

According to Zick, other birds, such the house sparrow, were forced to expand their range westward because their East Coast habitats could no longer support their growing population.

What about the unusual bird sightings? According to Zick, they’re a reflection of the birdwatchers’ ambitions, not range shift.

“Some bird watchers are really competitive,” Zick said. “They finally find these life-list birds and post the sighting online, when the bird is just passing through.”

Dale Humburg, chief biologist for Ducks Unlimited, a waterfowl conservation group, said analyzing this kind of data is never a simple issue.

“The key to understanding the meaning of this study,” Humburg said, “is that natural bird movement from year to year is so variable that it’s going to be pretty difficult to tease that apart from what may be a larger trend.”

Humburg said that in the short term, it’s difficult to discern the impact of climate change on birdlife. However, over decades, the results become clearer.

“I think if this continues,” Humburg said of global warming, “we could very well see a dramatic impact on birds. I see great potential threats under a changing climate.